Composite rotor blades conventionally are mechanically retained on a rotor hub by a root fitting or attachment which effectively transfers dynamic loading from the composite blade structure, through the fitting, to the rotor hub. In rotor blades of advanced composite structure, problems arise in transmitting the dynamic loads to the rotor hub. These loads include the torque, tension and bending loads on the rotor blades which must be transmitted to the rotor hub. In other words, the root attachment or fitting provides a path for loads from the composite structure to the rotor hub.
In the case of metallic blades which are retained for rotation upon the periphery of the rotor hub, a wide variety of mechanical fittings or attachment means are available and are quite effective. However, the trend toward incorporating composite blades into rotor assemblies has produced unique problems not heretofore experienced. By the term "composite blades" it is meant those blades formed by small diameter filaments of high strength, and high modulus of elasticity, embedded in a lightweight matrix. Typical examples are the nonmetallic composites such as graphite filaments with an epoxy resin matrix material, and certain metallic composites in a matrix such as aluminum.
The interface of composite materials of rotor blades and metallic elements of the rotor hub has been analytically and experimentally shown to be the crucial link in making composite rotor blade assemblies. This is the area where most problems occur, i.e. the area of transmitting the dynamic loads on the composite rotor blade to the metallic rotor hub. In the past, spade-type configurations have been used for the composite-to-metallic interface. Long fibers from the composite blade have been wound around the spade and bonded thereto with a resin. However, during fatigue life testing, the resin bonded to the metallic spade tends to separate and fail at that interface. The laminating of multiple plies of long fibers about the spade-type configurations can also be very time consuming hence costly. Winding of fibers around metallic elements may also not be possible where long fibers are not employed in the rotor blade materials.
Other attempts to solve the aforementioned problems are shown in U.S. Pat. Nos. 3,603,701; 3,731,360 and 4,031,601. U.S. Pat. No. 3,603,701 shows a system for transferring the dynamic loading forces from a composite rotor blade held between opposing external and internal metallic fittings, with a locknut in communication with a key to transmit torque from the internal fitting by a preloading scheme. U.S. Pat. No. 3,731,360 shows a method of making a composite blade with an integral root, in a compacting and bonding operation, and positioning a wedge between spread layers of the composite blade structure at the root end thereof. U.S. Pat. No. 4,031,601, discloses using a rather complicated dovetail slot interface between a fiberglass blade and a hub wherein, additional, substantially liquified resin and fiberglass material are forced through a bore in the hub into a slot therein.
Recent advances in ram air turbine technology have also lead to the use of longer rotor blades. In order to be able to use an existing hub and counterweight system, for example, it is necessary to reduce the weight of the longer blades in order to keep centrifugal loading within acceptable limits. For this reason, there is a need for an improved root attachment for a composite turbine blade of a ram air turbine which is free of mechanical fasteners such as bolts, rivets or studs for avoiding vibratory fatigue and subsequent stress concentrations and also for reducing weight, which improved root attachment can be secured to a composite turbine blade in a manner which resists centrifugal loads, aerodynamic bending loads due to pressure loading, centrifugal bending loads, and torsional loads.
U.S. Pat. No. 4,834,616 discloses a self-locking retention system for a composite rotor blade supported on a rotor hub wherein the composite thickness is woven in a pattern which grips the retention member in response to outward forces applied to the rotor blade and the woven composite thickness. U.S. Pat. Nos. 2,115,454; 3,883,267; 4,648,921 and 4,834,616 and also German Patent No. 3,015,208 concern blades and the provision of attachments therefor.